1. Field of the Invention
The present invention relates to a piezoelectric substance and a piezoelectric element using the piezoelectric substance.
2. Description of the Related Art
Piezoelectric substances are processed into various piezoelectric elements according to various purposes, and are utilized widely particularly as functional electronic components, such as an actuator, which applies voltage to cause deformation, a sensor, which conversely uses deformation of an element to generate voltage. As piezoelectric substances utilized in actuator and sensor applications, there have hitherto been widely used lead-based dielectrics possessing large piezoelectric properties, especially, Pb(Zr1-XTiX)O3-based perovskite-type ferroelectrics called PZT, which are formed typically by sintering oxides comprising individual elements. Also, in recent years, from environmental concerns, the development of piezoelectric substances that contain no lead has been desired, and lithium potassium sodium niobate (general formula: (NaxKyLiz)NbO3 (0<x<1, 0<y<1, 0<z<1, x+y+z=1), etc. has been developed. This lithium potassium sodium niobate (herein, referred to as LKNN) possesses as good a piezoelectric property as PZT, and is therefore expected as a major candidate for lead-free piezoelectric material.
On the other hand, at present, with reduction in size and enhancement in performance of electronic components of each kind, there is also a strong demand for size reduction and performance enhancement of piezoelectric elements as well. However, the thickness of piezoelectric material produced by a conventional manufacturing method using primarily sintering is particularly below 10 μm that approaches the size of crystal grains constituting the material, so the effect thereof becomes unnegligible. For that reason, there arises the problem of noticeable property variation and degradation. In order to avoid it, piezoelectric thin film-forming methods have, in recent years, been studied that apply thin film technology, etc. replaced for the sintering method. A PZT thin film formed by RE sputtering has recently been used practically as an actuator for heads of high-definition high-speed inkjet printers. In this piezoelectric thin film, there is also obviously a demand for lead-free piezoelectric material, as in the above piezoelectric substances.
As the related art, there is JP-A-2002-151754, for example.
As mentioned above, it is possible to produce very thin piezoelectric films, and therefore attempts to use the piezoelectric films in sensors are made at present. The piezoelectric films are deformed to produce voltage in proportion to their displacement (by producing charges in their surface). As sensors utilizing this phenomenon, there are gyrosensors, pressure sensors, etc. These sensors produce displacement as voltage where the voltage for no displacement is a reference. The gyrosensors receive Coriolis force, and also pressure sensors receive pressure, to cause displacement in the piezoelectric thin film. Accordingly, the piezoelectric thin film used in these sensors is required to be able to output sufficient voltage even for slight displacement. Such a piezoelectric thin film is a film having a fixed piezoelectric constant regardless of voltage where the piezoelectric constant is calculated by measuring displacement when applying voltage, utilizing the reverse piezoelectric effect.
The piezoelectric thin film is formed, for example, by sputtering, etc. We examine LKNN films that are lead-free piezoelectric substances. To make the above sensor using this thin film, a SiO2 film is formed on a low-cost silicon substrate. On that SiO2 film is formed an electrode of platinum (Pt), or the like. On that electrode is formed a piezoelectric thin film, using sputtering. The piezoelectric thin film having such structure formed is in a polycrystalline state.
The above piezoelectric thin film, like general ceramic piezoelectric substances, is prominent in having the dependence of piezoelectric properties on the magnitude of displacement but no linear relationship between displacement and voltage. Namely, only slight voltage is produced for small displacement, while large voltage is produced for small displacement. Such a film is a film having a small piezoelectric constant for small voltage and a large piezoelectric constant for large voltage when measuring displacement by applying voltage, utilizing the reverse piezoelectric effect. The piezoelectric thin film having such properties is not suitable for producing a high-sensitivity sensor.